1. Field of the Invention
The present invention relates to a capacitive pressure sensor and its manufacturing method and, more particularly, to a capacitive pressure sensor which simplifies a manufacturing process and improves yield.
2. Description of Related Art
In the manufacture of electronic components and semiconductor products, thin film deposition processes and etching processes are inevitably carried out in vacuum equipment. Such processes are generally carried out by keeping the pressure in the vacuum equipment constant with the aid of a pressure measuring means such as a capacitive vacuum sensor. Most commercial capacitive vacuum sensors have been manufactured by conventional machining; however a new type of pressure sensor and its manufacturing method by using micromachining techniques is proposed because it enables sensor miniaturization, mass production and cost reduction (JP2002-55008A, JP2001-201417A and JP2000-19044A). The micromachining technique makes use of semiconductor manufacturing techniques such as photolithography, film deposition, etching and the like. Moreover, commercially available materials such as a silicon wafer, a glass substrate and the like are used.
One example of conventional capacitive vacuum sensors manufactured by using a micromachining technique is shown in FIG. 4. FIG. 4A is a schematic exploded, perspective view showing a pressure sensor, and FIG. 4B is a schematic sectional view showing a pressure sensor which is connected to an electric circuit and placed in a case for practical use.
The pressure sensor is composed of a glass substrate 1, an SOI (Silicon On Insulator) substrate 2, and a glass substrate 3, which are fixed to each other in tight contact using a bonding technique. Here, the SOI substrate 2 generally consists of a silicon layer 4, a buried oxide layer 5, and a base silicon layer 6. A vacuum chamber groove 7 is formed in the silicon layer 4, and its closed space sealed with the glass substrate 1 is a vacuum. A capacitance electrode 8 and a reference electrode 9 are formed on the surface of glass substrate 1 which faces the vacuum chamber groove 7, and are respectively connected to a capacitance electrode terminal 10 and a reference electrode terminal 11 which are formed through the glass substrate 1.
The parts of base silicon 6 and buried oxide layer 5 which face the capacitance electrode 8 are removed to form a pressure-measuring chamber groove 12. As a result, the part of silicon layer 4 facing the capacitance electrode 8 acts as a diaphragm electrode 13. The diaphragm electrode 13 is electrically connected to a diaphragm electrode terminal 14 formed on the glass substrate 1.
Since the vacuum chamber must be kept at a high vacuum, a groove 27′ is formed in the glass substrates 1 and a non-evaporable getter, for example, is placed therein to absorb a residual gas inside the vacuum chamber 7. The pressure sensor is pressed on an O-ring 17 at the periphery portion of the glass substrate 3 by a press plate 16. Thus, the sensor is fixed on a base adaptor 18 with a vacuum seal.
A gas inlet 19 is formed in the central part of glass substrate 3 to make the pressure of pressure-measuring space 20 equal to that of the pressure-measuring chamber 12. Therefore, the diaphragm electrode 13 deflects depending on the pressure difference between the vacuum chamber 7 and the pressure-measuring space 20. The degree of diaphragm deflection can be obtained from the variation of electrostatic capacitance between the capacitance electrode terminal 10 and the diaphragm electrode terminal 14. Here, a reference electrode 9 is formed in the vicinity of the capacitance electrode 8 in order to correct the error due to the mechanical distortion which is caused when the ambient temperature changes because of the difference in the thermal expansion coefficient of sensor components.
In addition, a plurality of small projections 23 are formed on the diaphragm electrode 13 in order to prevent the diaphragm electrode 13 from sticking to the capacitance electrode 8 when the pressure of the pressure-measuring space 20 becomes high and the diaphragm comes in contact with the capacitance electrode.
Next, the manufacturing process of the pressure sensor shown in FIG. 4A is explained by referring to FIG. 5.
First, an SOI substrate 24 for forming the capacitance electrode and the reference electrode is prepared (FIG. 5A). An oxide film 25 is formed on the surface and patterned (FIG. 5B). Then, the exposed portions of the silicon layer 4 and the buried oxide layer 5 of the SOI substrate 24 are removed (FIG. 5C). The glass substrate 1 in which grooves 26 for electrode terminal and a getter chamber groove 27′ are formed is bonded with the SOI substrate 24 (FIG. 5D), and thereafter only the base silicon layer 6 is etched and removed (FIG. 5E).
Similarly, an oxide film 25 is formed on the SOI substrate 2 and patterned (FIG. 5F). A silicon layer 4 is partially etched (FIG. 5G). Then, the oxide film 25 on the upper surface is patterned again, and the silicon layer 4 is etched until it comes to the thickness of diaphragm electrode (FIG. 5H). Then, after the oxide film on the lower surface of base silicon layer 6 is patterned, small projections 23 are formed using a metal material such as aluminum or a silicon oxide film on the region which is to be the diaphragm electrode (FIG. 5I).
Next, the glass substrate 1 and the SOI substrate 2 are aligned, placing a getter 15 inside the getter chamber groove 27′ formed in the glass substrate 1, and then bonded in a vacuum by the anodic bonding method (FIG. 5J). After that, the base silicon layer is etched until the buried oxide layer is exposed (FIG. 5K). The glass substrate 1 is etched by using, for example, a hydrogen fluoride solution to expose the capacitance electrode 8 and the reference electrode 9 inside the grooves 26 for electrode terminal, while the buried oxide layer 5 underneath the diaphragm electrode 13 and the oxide film 25 on the base silicon layer 6 are removed at the same time. Thereafter, metal electrodes 28 are formed inside the grooves 26 for electrode terminal and on the surface of glass substrate 1, which are respectively connected to the capacitance electrode 8, the reference electrode 9, and the silicon layer 4 (FIG. 5L). Then, the glass substrate 3 with gas inlet 19 is bonded with the base silicon layer 6 of the SOI substrate 2, and finally the capacitance electrode terminal 10, the reference electrode terminal 11, and the diaphragm electrode terminal 14 are connected to terminal pins 30 using a conductive adhesives 29 to complete the pressure sensor (FIG. 5M).